Question

Some of the following gas molecules have infrared absorption spectra and some do not: $\mathrm{N}_{2}, \mathrm{HBr}, \mathrm{CCl}_{4}, \mathrm{CH}_{3} \mathrm{CH}_{3}$ $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}, \mathrm{H}_{2} \mathrm{O}, \mathrm{CO}_{2}, \mathrm{O}_{2} .$ What is the gross selection rule for vibrational spectra, and which molecules satisfy it?

Name: Problem 61, Chapter 13: Physical Chemistry
Uploaded: 2024-01-22T00:44:01-08:00
Channel: Andrew Eddins
Description: Some of the following gas molecules have infrared absorption spectra and some do not: N2, HBr, CCl4, CH3 CH3 CH3 CH2 OH, H2 O, CO2, O2 . What is the gross selection rule for vibrational spectra, and which molecules satisfy it?

   Some of the following gas molecules have infrared absorption spectra and some do not: $\mathrm{N}_{2}, \mathrm{HBr}, \mathrm{CCl}_{4}, \mathrm{CH}_{3} \mathrm{CH}_{3}$ $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}, \mathrm{H}_{2} \mathrm{O}, \mathrm{CO}_{2}, \mathrm{O}_{2} .$ What is the gross selection rule for vibrational spectra, and which molecules satisfy it?

Physical Chemistry

Robert J. Silbey,… 4th Edition

Chapter 13, Problem 61

Instant Answer

Solved by Expert Andrew Eddins

01/22/2024

Step 1

The gross selection rule states that a molecule must have a changing dipole moment during its vibration in order to have an infrared absorption spectrum. In other words, the molecule must be polar and its vibration must cause a change in its dipole moment. Now, Show more…

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Some of the following gas molecules have infrared absorption spectra and some do not: $\mathrm{N}_{2}, \mathrm{HBr}, \mathrm{CCl}_{4}, \mathrm{CH}_{3} \mathrm{CH}_{3}$ $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}, \mathrm{H}_{2} \mathrm{O}, \mathrm{CO}_{2}, \mathrm{O}_{2} .$ What is the gross selection rule for vibrational spectra, and which molecules satisfy it?

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Key Concepts

Selection Rule Based on Dipole Moment Change

A key rule in infrared spectroscopy is that a vibrational mode must involve a change in the dipole moment of the molecule in order to be IR active. This means that even if a molecule vibrates, if the vibration does not alter the dipole moment, the mode will not absorb infrared radiation. Molecules with no permanent dipole moment, or vibrational modes that do not cause a dipole change (e.g., symmetric stretching in certain molecules), typically do not show IR absorption.

Vibrational Infrared Spectroscopy

This technique involves the absorption of infrared light that causes transitions between the vibrational energy levels of a molecule. The ability of a vibration to interact with electromagnetic radiation in the IR region depends on various factors including the molecular structure and symmetry, making it a powerful tool for molecular identification and structural analysis.

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